its not that it wasn't thought of , its just not really worth it . i did it when i put a 94 W motor in a 88 Z motor astro . it was easy for me cuz all i had to do was swap out motors , all the electrics were allready there . the difference in the harness & computers & sensors to change would be a real PIA. & you wind up with less horse power besides , granted the TBI is more reliable , but not worth the change over .
to put a 5.7 TBI in a astro with CPI is not all that dificult but you will need the harness computer & sensors from a v8 or v6 TBI system .